Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEMs also allow analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite side. Samples, also referred to as lamellae, are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. The term “TEM” as used herein refers to a TEM or a STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on a STEM. The term “S/TEM” as used herein also refers to both TEM and STEM.
Bright field imaging and dark field imaging are often used in the context of S/TEM imaging. A bright field image may be formed by selecting electrons from a central diffraction spot to form the image. In other words, bright-field images are formed by detecting the electrons that pass through the sample without significantly scattering as they pass through the sample. In contrast, a dark field image may be formed in the S/TEM by using some or all of the non-central (diffracted) electrons. The dark-field images are obtained by detecting the electrons that scatter as they pass through the sample.
FIGS. 1A and 1B show steps in thinning and imaging a TEM sample according to the prior art. The sample 20 is typically attached to a TEM sample holder 24 and thinned using a focused ion beam 22 (FIB). The sample can be imaged with an electron beam 25 in a TEM or STEM as shown in FIG. 1B. A typical dual beam FIB/SEM has the SEM column oriented normal to the sample (at 90 degrees) and the FIB column at an angle of approximately 52 degrees. It is often desirable to image the sample during milling using the SEM. Imaging using SEM or S/TEM during sample thinning allows the sample thickness and location of the feature of interest within the sample to be monitored directly. S/TEM imaging can be used even when the sample surface is at an angle to the electron beam (as would be the case when the sample is oriented toward the ion beam during milling) by compensating for the angle mathematically.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. Accurately determining the endpoint for lamella creation is becoming even more difficult as devices grow in complexity and shrink in size. Smaller features can require smaller and thinner S/TEM samples. In many cases, samples are thinned using a focused ion beam system. It is often very difficult to determine when the sample has been sufficiently thinned. If samples are left too thick they won't be sufficiently transparent to the electrons for S/TEM analysis. On the other hand, if the sample is thinned too much, the features to be measured or even the entire sample may be destroyed. Even for a sample that is within the acceptable range of thickness, variation between samples is undesirable.
Consequently, precise endpoint detection for lamella thinning is very important. Historically, the TEM sample preparation process has been performed using instruments operated manually. Attempting to determine the precise endpoint for sample thinning (i.e., endpointing) is typically more of a guess than an actual calculated endpoint determination. For this reason, successful S/TEM sample preparation generally requires the use of highly trained and experienced operators and technicians. Even then, it is very difficult to meet any reasonable standards of reproducibility and throughput. Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control.
What is needed is a method of precisely calculating an endpoint for FIB sample thinning to use in TEM sample creation. What is also needed is a method that lends itself to automation to increase throughput and reproducibility so that TEM measurement can be incorporated into integrated or in situ metrology for process control.